It is well known in the luminaire art to combine a paraboloidal reflecting surface with a light source located at the focal point of the said reflecting surface to project light rays in parallel paths of travel, thus producing a spot configuration of reflected light. It is also known that a sleeve member of translucent or frosted material will, when interposed between the light source and the reflecting surface, provide a flood configuration of reflected light due to the scattering of the light rays as they pass through the material of the sleeve member. U.S. Pat. No. 1,991,753 (Kurlander) illustrates such a device wherein the inventor specifies that a translucent material must be used. In this invention a sleeve member is moveable in a direction parallel to the central axis of a reflector to provide an adjustment of distribution between spot and flood. In addition, a translucent or frosted sleeve member having angular serrations on an outer surface is disclosed.
A translucent or frosted sleeve arrangement such as that described in the above referenced patent, however, presents certain disadvantages. Firstly, the use of a translucent or frosted material for the sleeve member of such an arrangement is quite costly in terms of luminaire efficiency; such materials diffuse light by a principle known as "scattering". In the case of a translucent material the scattering effect is caused by the presence of tiny opaque or reflective particles suspended in the medium of the material. A light ray entering such a material is redirected by reflection each time it encounters such a particle; rays emerging from the material, therefore, have been reflected from and redirected by many such particles, and a portion of the light energy of each ray has been absorbed by each such encounter. In addition, some rays will be redirected backward (toward the light source), a phenomenon known as "backscattering", and thus will not emerge in the direction of the reflecting surface. Further, the complex path of travel produced by a myriad of such encounters will cause some rays to be permanently lost within the material; that is, the number of encounters may be sufficiently great as to absorb virtually all of the light energy of these rays. A prime example of a translucent material is fog; clearly this is a translucent material in which the phenomena of scattering, backscattering (experienced as glare) and absorption may be seen.
Frosted materials exhibit similar effects, albeit for slightly different cause. In this case at least one surface of a nominally transparent material is treated such that these surfaces become etched or microscopically pitted. These pits are partially opaque or reflective and act in much the same way as the aforementioned particles; light is redirected by scattering, it may be backscattered and it may be absorbed.
Clearly, scattering will cause light to emerge from such a material in a totally random fashion. In fact, observation of a standard frosted light bulb will indicate that the entire surface of such a material becomes a "secondary emitter" of light; that is, the entire surface appears to be luminous. Control of distribution of such light with a paraboloidal reflecting surface is nearly impossible because of the random angles of incidence of light upon the reflecting surface. This is definitely a second disadvantage.
The addition of angular serrations to one or more surfaces of a translucent or frosted sleeve member has no effect other than to increase the degree of scattering. Indeed, such serrations may further lower the transmissive efficiency of the sleeve member since backscatter and absorptive effects may be increased.
A third disadvantage of the above referenced invention is the requirement for a relatively large hole in the reflector surface concentric with the central axis in order to permit travel of the sleeve member (to allow adjustment between spot and flood). This requirement reduces luminaire efficiency, particularly in spot adjustment, due to the loss of light through this hole.